1. Field of the Invention
The present invention generally relates to an optical structures and methods of making such structures, and more particularly, to an optical structures including attenuators and modulators which vary the intensity of a guided lightwave by controlling the amount of power coupling between a guided light mode and a surface plasmon wave mode.
2. Description of Background Art
A surface plasmon wave (SPW) is an electromagnetic wave which propagates along the interface between two materials having dielectric constants of opposite signs, e.g., a metal and a dielectric layer. The polarization of a SPW is transverse magnetic (TM) since its electric field is perpendicular to the propagation interface. SPWs can be analyzed by techniques used for TM optical modes since they obey identical field equations and satisfy the same boundary conditions. Unique features of SPWs are that almost all of their energy is concentrated at the dielectric/metal interface and their propagation characteristics are very sensitive to environmental changes in their proximity.
Optical plasmon wave structures can be made by employing a device which converts an optical guided lightwave into a SPW. By controlling the amount of power coupling between the optical signal and the SPW, optical power may be dissipated producing an attenuation of the optical signal. If the amount of coupling is controlled by the physical design of the device, an attenuation device can be provided. If the amount of coupling is controlled electrically, a variable attenuator (modulator) device can be provided.
Because SPWs are very sensitive to environmental changes, introducing an additional dielectric (or polymer) layer with electrically variable refractive index (an electro-optic or EO layer) in contact with the metal layer supporting the SPW will cause variations in the propagation constant of the SPW. These propagation constant variations result in power coupling variation between the SPW and the optical wave and consequently modulation of the optical wave. The refractive index variation of the dielectric/polymer layer can be accomplished via optics modulator configuration situated on the top of the layer.
Because of the extremely small interaction lengths needed, the optical plasmon wave modulator can be a very compact device which can be implemented as an integrated optics structure. This small size and other advantages of the device can produce substantial benefits with its use in the electronics and communications areas.
However, attempts to develop a practical optical SPW modulator which could be commercialized have not been successful. Other technologies, for example electro-optic (EO) modulators and semiconductor absorption modulators continue to dominate devices which have been commercialized. There is only one demonstration of planar integrated optics SPW modulator in Novel Integrated Optical Intensity Modulator Based On Mode Coupling by Driessen, et al., Fiber Optics, Vol. 13, pp 445-461, 1994. Another experimental approach is shown in Integrated Optics Waveguide Modulator Based On Surface Plasmon Resonance by Jung, et al., Journal of Lightwave Technology, Vol. 12, No. 10, October 1994. Two theoretical examples of integrated optics, one planar and the other cylindrical (fiber), SPW modulators are shown in U.S. Pat. No. 5,067,788 issued Nov. 26, 1991 to Jannson, et al. The disclosure of Driessen, et al.; Jung, et al. and Jannson is hereby incorporated by reference. All of foregoing presently exhibit inferior performance, manufacturability and design flexibility to the alternative technologies.
All of the above mentioned disclosures use a generic configuration for SPW generation and modulation control which consists of an EO dielectric (or polymer) layer sandwiched between two metal layers. The SPW is supported in the bottom metal layer of the sandwich and a modulation voltage is applied between the two metal layers to change the refractive index of the EO layer. This combined SPW generation/control configuration, which behaves like a parallel plate capacitor, is attached on the optical structure supporting the guided optical mode. The application of voltage at the electrodes changes the refractive index of the EO layer and consequently the SPW properties, i.e., its propagation constant .beta..sub.spw, and the coupling coefficient between the SPW and the optical mode.
The deficiencies of this sandwich design are caused primarily by an attempt to combine the modulator control structure with the surface plasmon wave generation and support structure. The combination of these two functions puts severe design constraints on the thickness of the metal layer in which the SPW must be generated. Because electric current penetration is directly related to its frequency (the skin effect), the thickness of the modulator electrodes must always be greater than the required skin depth at the corresponding maximum modulation frequency to avoid high skin resistance. High resistance increases the input voltage for a desired modulation depth and generates heat which is highly undesirable. If intense enough, such heating may damage the metal layer or the EO layer with which the metal layer may be in contact. Particularly vulnerable to heat damage are thin polymer EO layers. Further, the appropriate electrode layer thickness for a given bandwidth may be thinner or thicker than the necessary thickness to support a SPW with the desired propagation constant, .beta..sub.spw. These two design constraints are often in conflict and may prevent a pragmatic solution at the modulation bandwidth desired and at the wavelength desired for the guided wavevector. Still further, the use of this structure as a modulator is limited because the modulation electrodes act as a parallel plate capacitor. This electrode configuration has high capacitance which greatly limits the overall bandwidth.
Moreover, one of the more useful optical SPW devices would be an wavelength sensitive attenuator or modulator which can be used in an cylindrical integrated optics geometry, such as an optical fiber, where the core of the optical fiber would provide the guiding layer for the guided light wavevector. Because optical fibers are designed for minimal power loss and dispersion at a particular wavelength, their core and cladding material and core thickness are parameters which must be used as a given in the design of optical SPW devices which interface with them. These constraints make it even more difficult to design efficient SPW generation and support structures, particularly if modulation electrode constraints must be simultaneously satisfied.